Noise and vibration from HW system when CH is on.

[Grogey]

Hi all,

In the hotpress, I'm getting noise and vibration at the 'cold return' pipes heading to the boiler from the hot water cylinder, heating coil.
This strangely happens when the the CH is ON but when the hot water heating is OFF.

The vibrations can be felt when holding the pipes where they disappear under the hotpress floor. I am definitely not confusing the CH pipes with the HW pipes. The noise can be heard throughout the house.

Two motorised valves located adjacent the hot water cylinder distribute the boiler feed to either the cylinder or the radiators. Activating the HW system stops the noise.

I'm assuming air in the system is the cause but can't figure out a connection between the CH and HW.

Any ideas before I call my plumber. I'm struggling to get a plumber to call out for minor jobs, especially one that is more irritation than major fault. They are all very busy with big jobs. Any info that could speed resolving this would help us both.

Cheers

Grogey